1. Field of the Invention
The present invention relates to a high toughness, low yield ratio, high fatigue strength steel plate and a process of producing same.
2. Description of the Related Art
In recent years, demand has been increasing for steel plates having a low yield ratio and high tensile strength ensuring the good earthquake-resistance required for plates, pipes, sections and other steel members used for constructing buildings or other structures.
In response to this demand, processes were proposed in Japanese Unexamined Patent Publication (Kokai) Nos. 55-41927 and 55-97425. The former uses controlled rolling and controlled cooling whereas the latter uses quench-hardening and tempering. Both of these processes provide a steel having a 60 kgf/mm.sup.2 -grade tensile strength and do not generally produce steels having high strength.
Other manufacturing processes have been also developed, such as those described in Japanese Unexamined Patent Publication (Kokai) Nos. 63-293110 and 59-211528. In these processes, a hot-rolled steel plate is allowed to cool naturally to a selected temperature and is then rapidly cooled so as to establish a metallographical microstructure in which the ferrite phase is mixed with bainite and/or martensite phases to provide a low yield ratio. In these processes, however, the ferrite phase is generated during the natural or air cooling and has a coarse grain size which results in poor toughness. Japanese Unexamined Patent Publication (Kokai) No. 63-286517 disclosed another process in which a hot-rolled steel is heat-treated in a dual-phase temperature region higher than the A.sub.C1 temperature of the steel. This process effectively lowers the yield ratio, but has a problem that such a high temperature heat treatment induces the hot-rolled microstructure to coarsen by recrystallization and transformation, which results in poor toughness.
Marine vessels, offshore structures, bridges, and land-based structures are often subjected to fluctuating load, so that weld bonds and unavoidable structural stress concentrators generate fatigue cracks which propagate and cause the structure to fracture. Many measures have been taken in designing and construction of these structures in order to avoid stress concentration, as proposed in Japanese Examined Patent Publication (Kokoku) No. 54-30386. However, this imposes significant limitations on the design and construction and/or requires increase in the structure weight. Therefore, it has been desired to improve the fatigue strength of a steel member, and not rely upon design or construction.
To improve the fatigue strength of a steel member, processes were proposed in Japanese Examined Patent Publication (Kokoku) No. 3-61748 and Japanese Unexamined Patent Publication (Kokai) No. 3-291355. The process of the former, however, is only applicable to steels having relatively high carbon contents such as strengthening parts of automobiles, and is not suitable for the above-recited structures. The process of the latter also has a drawback of limited application due to an uneven distribution of properties through the thickness of the steel.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 64-79345 disclosed a steel containing retained austenite phase, in which the TRIP phenomenon is utilized. This steel is basically used in the form of a sheet imparted with good combination of strength and ductility, but not imparted with improved fatigue strength. Steel microstructures containing retained austenite were also discussed in Japanese Unexamined Patent Publication (Kokai) Nos. 58-139656, 60-5820, 60-17013, 60-43425, 60-165320, 63-4017, 63-282240 and 3-10049, which all were intended to improve the toughness and workability of hot- or cold-rolled steel sheet and do not provide a steel plate having high fatigue strength, which is an object of the present invention.
Japanese Unexamined Patent Publication (Kokai) No. 57-108241 disclosed a steel having a mixed microstructure containing martensite phase, specifically a ferrite-martensite mixed microstructure, which is basically used in the form of a sheet having good combination of strength and elongation, but not having improved fatigue strength. Similar steels are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 57-137542 and 58-6937. Japanese Unexamined Patent Publication (Kokai) Nos. 58-93814 and 62-174322 disclosed steel plates having a composite microstructure containing martensite phase, which are intended to provide a low yield ratio but do not provide improved fatigue strength.